Porous carbon fiber substrates play a pivotal role in enhancing the performance of potassium–selenium (K–Se) battery, as they are key to efficiently confining small Se molecules and regulating selenide reactions. Herein, to address the issues of volumetric expansion and polyselenide formation caused by disordered Se aggregation, a three-dimensional composite material of microporous carbon nanofibers coated with Se and Fe was synthesized, effectively suppressing the shuttle effect of liquid-phase polyselenides. State-of-the-art analytical techniques and theoretical modeling were employed to elucidate the deposition dynamics of Se molecules within micropores and the mitigation mechanisms for volumetric expansion effects. The pore-confinement effect optimizes the spatial distribution of Se, while Fe catalyzed the K₂Se_x, effectively mitigating the sluggish reaction kinetics associated with K₂Se_x formation. This study offers a theoretical framework and experimental insights to advance the development of K–Se battery with high energy density and superior cycling stability.